Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2002-01-17
2004-08-24
Tran, Khai (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
Reexamination Certificate
active
06782060
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for generating reliability information for a channel decoder of a radio receiver, particularly a mobile radio receiver, and to a corresponding radio receiver.
Transmission channels in mobile radio systems are characterized by their time-dependent multipath reception, which leads to intersymbol interference in the case of digital transmission systems. In order to be able to control such intersymbol interference, equalization of the received data is required at the receiving end. At the transmitting end, the data to be transmitted are transmitted interleaved and channel-coded due to the rapidly changing transmission conditions and for suppressing adjacent-channel and co-channel interference.
For the channel decoding at the receiving end, it is desirable to have information that specifies the reliability of the equalization performed by the equalizer. This reliability information is information that is obtained by a so-called soft decision. In contrast to a hard decision in which only a fixed decision threshold is used, a multiplicity of decision thresholds is used in the case of a soft decision, which distinctly increases the reliability of the decision. Equalizers as used, for example, in GSM receivers and also provided in accordance with the future expansion of the GSM mobile radio standard, EDGE, therefore must adequately equalize the received signal on the one hand, and, on the other hand, provide the aforementioned reliability information.
Many different methods for generating the aforementioned reliability information are known and in mobile radio systems, algorithms are frequently used which are based on a so-called Maximum Likelihood Sequence Estimation (MLSE) described, for example, in “Digital Communications”, Proakis, J. G., McGraw-Hill, New York, 1983. The most widely used implementation of this method is the Viterbi algorithm with which the aforementioned reliability information is obtained with the aid of trellis diagrams, in the form of probabilities of whether a received symbol is based on a transmitted ‘0’ or a transmitted ‘1’.
However, since this (optimum) algorithm is very complex and, as a result, very computationally intensive, and requires very large storage space, various sub-optimal methods have been developed that provide reliability information for the channel decoder in a simpler manner.
Such a sub-optimal method is described, among other things, in “Optimum and Sub-optimum Detection of Coded Data Disturbed by Time-Varying Intersymbol Interference”, Wolfgang Koch and Alfred Baier, 1990 IEEE. According to this method called “Reduced State Soft Decision Equalizer”, the reliability information is generated symbol by symbol in the equalizer. The corresponding algorithm is very similar to a hard decision Viterbi algorithm but it generates the reliability information in a distinctly simpler manner, the reliability information for a received symbol at time &mgr;−L being determined at a time &mgr;. L here designates the length of the observation period and corresponds to at least the length of the channel impulse response of the transmission channel. The reliability information is determined by determining the best “one path” of the trellis diagram (i.e., the best or most advantageous path having the value ‘1’ at time &mgr;−L) and the best “zero path” (i.e., the best or most advantageous path having the value ‘1’ at time &mgr;−L) by means of a trellis diagram. These two paths of the trellis diagram are determined by means of metrics that are calculated for the individual state transitions in the trellis diagram. In this method, in particular, the so-called “matched-filter” metric is used. Finally, the reliability information is obtained by putting the metrics calculated for the best “one path” and best “zero path” in this manner in relation to one another. To reduce the computational expenditure and the storage requirement, a trellis having a reduced number of states is used for calculating the individual metrics. A trellis-based equalization is only started for elements 0 . . . L′ (L′<L) of the channel impulse response whereas the remaining elements L′+1 . . . L are only included in the trellis-based equalization in a decision-feedback manner. The principles of this decision feedback (Decision Feedback Sequence Estimation) can be found, for example, in the paper “Reduced-state Sequence Estimation with Set Partitioning and Decision Feedback”, Vedat Eyuboglu and Shahid Qureshi, 1988 IEEE.
In the procedure described above, the equalizer must determine the best “one-path” and “zero path” with reference to time &mgr;−L′ at each time &mgr; and calculate from these determinations the reliability information for the received symbol at time &mgr;−L′. In this process, the branch metrics include bits at times &mgr; . . . &mgr;−L′ and bits at times &mgr;−L′−1 . . . &mgr;−L, the latter bits, as already described, being included in the metrics calculation as decision feedback. These latter bits are obtained from the individual so-called “survivor” paths of the 2
L′
states of the trellis diagram (i.e., the most inexpensive and most probable state transitions in each case), which, however, are different from state to state as a consequence, so that a correspondingly high computational effort and storage requirement is needed since the equalizer must carry a list with 2
L′
states at each time &mgr;.
SUMMARY OF THE INVENTION
The presently disclosed method and apparatus provide generation of information for channel decoding in a radio receiver wherein the computational expenditure and the storage space needed for computations are reduced.
Specifically, a method for generating reliability information for channel decoding in a radio receiver is disclosed wherein reliability information (q) specifies probabilities of a data symbol (z) received by the radio receiver via a radio channel based on one of first and second values that are transmitted. The method includes determining reliability information (q) for a time &mgr;−L′ at an arbitrary time &mgr;for each received data symbol (z) by determining through the use of a state model having 2
L
′ states a first path that most probably contains the first value at time &mgr;−L′. Also a second path which most probably contains a second value at time &mgr;−L′ is determined and metrics calculated for the first path and the second path are placed into a relationship with one another wherein metrics calculated for the first path and the second path are calculated in dependence on a first group of symbols of the state model present at time &mgr; . . . &mgr;−L′ and a second group of symbols of the state model present at times &mgr;−L′−1 . . . &mgr;−L and L corresponding to at least the length of a channel impulse response of the radio channel with L greater than L′. Furthermore, the method includes the step of utilizing a value that has been decided before the time &mgr;−L′ and is identical for all states of the state model as symbols of the second group for determining the reliability information (q) for time &mgr;−L′.
An apparatus for use in a radio receiver is also disclosed that includes an equalizer configured to equalize a radio signal received via a radio channel and generate reliability information (q) for a downstream channel decoder, wherein the reliability information (q) specifies probabilities of a received data symbol (z) based on at least one of a first and second transmitted value. The equalizer also determines for each received data symbol (z) at an arbitrary time &mgr; a reliability information item (q) for a time &mgr;−L′ by determining, by means of a state model with 2
L′
states, a first path that most probably contains the first value at time &mgr;−L′ and a second path that most probably contains the
Bohnhoff Peter
Hartmann Ralf
Infineon - Technologies AG
Tran Khai
LandOfFree
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